1. Field of the Invention
The present invention relates to a VA (vertically aligned)-mode liquid-crystal display device improved in the front contrast ratio.
2. Background Art
These days elevation of the contrast ratio (CR) in liquid-crystal display devices is being promoted. In particular, a VA-mode liquid-crystal display device has the advantage that CR in the normal direction (hereinafter referred to as “front CR”, and “front CR” may be generally called also as “on-axis contrast ratio”) is high as compared with that in other modes, and various studies and developments are now made for further enhancing the advantage. As a result, in these 6 years, the front CR in VA-mode liquid-crystal display devices has increased from about 400 to about 8000, or by about 20 times.
The front CR is an important property that is to be the index of image sharpness, and a VA-mode LCD is characterized by high front CR and is now the mainstream of LCD panels.
On the other hand, in liquid-crystal display devices, it is important that not only the front CR is high but also the panel secures a broad viewing angle (that is, CR in oblique directions (hereinafter this may be referred to as “viewing angle CR”) is high), and that the panel is free from color shift in oblique directions. For improving the viewing angle characteristics of liquid-crystal display devices, in general, a retardation film is disposed on the front side and the backside of the liquid-crystal cell.
For example, in a VA-mode liquid-crystal display device, in general, a retardation film is disposed each one on the front side and on the rear side, thereby distributing retardation necessary for viewing angle compensation into each one to compensate the viewing angle on the panel. There are known an embodiment where a film having the same retardation is disposed both on the front side and on the rear side (hereinafter this may be referred to as “both-sides retardation film type”); and an embodiment where an inexpensive film such as a plain TAC film or the like is disposed on any of the front side or the rear side, and a film having a large retardation is disposed on the other side (hereinafter this may be referred to as “one-sides retardation film type”).
The former embodiment is advantageous in that one and the same retardation film can be used; and the latter embodiment is advantageous in that a popular film can be used on one side.
For increasing the front CR of a liquid-crystal display device, the haze of the retardation film to be used for viewing angle compensation is preferably lower (for example, JP-A 2007-133379). In particular, as described in the above, a VA-mode liquid-crystal display device has a higher front CR than liquid-crystal display devices of other modes, and the request for the device of preventing the front CR reduction to be caused by the retardation film therein is higher than that for liquid-crystal display devices of other modes.
Popular retardation films are inexpensive, and use of such popular films for retardation films is preferred for reducing the production cost of liquid-crystal display devices. On the other hand, for reducing the haze of retardation films, severe control of production conditions and limitation on additives and starting polymers are necessary, or that is, the haze reduction is accompanied by cost increase in some degree. Accordingly, almost all inexpensive popular retardation films are not processed for haze reduction, or that is, there are a lot of popular retardation films having a high haze.
The front CR of existing liquid-crystal display devices is low; and in these devices, even when popular retardation films are used and the front CR is reduced in some degree owing to the haze thereof, the degree of front CR reduction is small and is on an unrecognizable level. However, as described in the above, with the recent tendency toward higher-CR liquid-crystal display devices, even some slight influence to be caused of the haze of retardation films could appear as a significant change in the front CR.